This invention relates to free piston Stirling machines and more particularly relates to a free piston Stirling machine that is adapted for use in applications where its component parts that are in the region of its expansion space are subjected to extreme temperatures.
Free piston machines, including free piston engines, coolers and heat pumps, have been applied to a variety of purposes in a variety of environments. Typically they have a compression region of the machine that operates at a temperature that is nearer to their ambient temperature and an expansion region that operates at a temperature that is farther from their ambient temperature. The expansion region is usually at one end of a generally cylindrical head and is either much colder than the ambient temperature, as in the case of a cryocooler, or the expansion region is much hotter than the ambient temperature, as in the case of a engine.
These temperature extremes present difficult design problems because of the temperatures themselves and because of the temperature differential between the component parts in these regions and the parts in the remainder of the machine. Some component parts extend into both the region of extreme temperature and the region of more moderate temperature. Typical problems include selecting materials that can maintain their characteristics and function properly at the extreme temperature and selecting the dimensions of the component parts and selecting machining tolerances to accommodate thermal expansion and contraction of the materials.
The design of a free piston Stirling engine for use in interplanetary travel is an example of the need to contend with extreme temperatures. The temperature of the atmosphere on Venus is on the order of 500° C. Because heat is applied to the heater head of an engine to power the engine, for use on Venus there is a need for a heater head that can withstand on the order of 1100° C. One of the most difficult component parts to design in a manner that can accommodate the extreme temperature is the displacer of the free piston Stirling machine. The reason is that the displacer of a free piston Stirling machine not only reciprocates along an axial path between the compression space with the moderate temperature and the expansion space with the more extreme temperature but the displacer also extends essentially all the way from within a heat rejecting heat exchanger at the compression space, through a regenerator to within a heat accepting heat exchanger at the expansion space. Consequently, the reciprocating displacer is subjected to an extreme temperature differential between its opposite ends with a temperature gradient along its length.
Efficient work is done in a free piston Stirling machine by transferring heat into the working gas at the expansion space and transferring heat out of the working gas at the compression space. Heat that is instead transferred through the displacer is wasted or lost heat representing inefficiency. Therefore, displacers are designed to minimize the heat transfer through the displacer from one end to its opposite end. Consequently, a typical displacer has a thin walled dome at its expansion space end fixed on top of a more rigid supporting piston at its compression space end. Examples of such displacers are illustrated in U.S. Pat. Nos. 4,559,779 and 7,866,153. The dome typically has an axial length that is considerably longer than its rigid supporting piston and its purpose is to thermally isolate the hot and cold spaces (expansion and compression spaces). The dome is a thin walled and essentially hollow structure in order to minimize its mass and to minimize heat conduction through the metal of the displacer. The displacer usually has baffles in the interior of the displacer dome to function as radiation shields and to subdivide the space in order to limit gas convection within the displacer and thereby limit heat transfer through the displacer between the expansion space and the compression space. Typically there are 3 to 6 baffles tack welded inside the displacer. Such displacers are expensive to manufacture and subject to thermal expansion/contraction. The metal, especially of the dome, must be able to withstand the extreme temperatures of the expansion space. Furthermore, the baffles can also be a reliability problem, especially if they become detached from the interior wall of the displacer dome.
An example of a typical prior art free piston Stirling machine 8 is illustrated in FIG. 2. The machine 8 has a hermetically sealed casing 10 containing a displacer 12 connected to a connecting rod 14 that is attached at its opposite end to a planar spring 16. The displacer reciprocates along a central axis 17 within a displacer cylinder 18 and extends between an expansion space 20 and a compression space 22. The displacer cylinder 18 extends within a heat accepting heat exchanger 24, a regenerator 26 and a heat rejecting heat exchanger 28 all of which surround the displacer cylinder 18 and permit working gas to be shuttled between the expansion space 20 and the compression space 22 serially through the heat exchanger 24, regenerator 26 and heat exchanger 28. The working space of the Stirling machine 8 is bounded by a piston 30 that reciprocates in a piston cylinder 31 and is connected to magnets 32 of an electromagnetic linear alternator/motor 34. The time varying pressure within the working space drives the reciprocation of the piston and the displacer.
The prior art has suggested avoiding the problems of the extreme temperature at the expansion space end of a displacer by using a thermoacoustic Stirling heat engine configuration. This configuration eliminates the displacer and substitutes a tuned inertance tube. Consequently it has no moving part that extends to the extreme temperature of the expansion space end of the head. Typically the inertance tube is ¼λ long and extends through a radial port in a generally radial direction out the side of the machine at the heat rejecting, compression space end of the working gas space and returns to the compression space through another radial port.
This thermoacoustic solution, however, introduces several disadvantages. The thermoacoustic Stirling heat engine configuration has a lower efficiency than a Stirling machine using a displacer because of the less than ideal phasing of the working gas through the regenerator and the added gas volume in the inertance tube. Another disadvantage is that a thermoacoustic Stirling heat engine requires a fluid diode for preventing a detrimental, unidirectional, circulating fluid flow component of working gas. There is also the problem of attaching the inertance tube to the casing in a manner that is durable and provides proper gas communication with the compression space. The inertance tube also forms an unwieldy arm that projects out the side of the machine.
It is therefore an object and feature of the present invention to provide a free piston Stirling machine that avoids problems inherent in the presence of extreme hot or cold temperatures to which component parts in the expansion region of a free piston machine are subjected.
Another object of the invention is to avoid the problems presented by the extreme temperatures and yet retain a displacer in the machine so that the higher efficiency of a free piston machine that has a displacer can be attained and the disadvantages of the inertance tube and fluid diode of the thermoacoustic configuration can be avoided.